| High pressure air impact fracturing is a new method of coal and rock fracturing with wide application prospect,which can be applied to various coal and rock fracturing engineering in underground.However,the dynamic failure characteristics and fracture morphology of coal rock caused by high-pressure air impact are not clear,which limits the application of high-pressure air impact fracturing technology.Aiming at this problem,constructing the high pressure air shock crack coal and rock true triaxial test platform,to develop the coal and rock block high pressure air impact crack test,explores the direction of the shock pressure,borehole coupling coefficient,and stress are the key factors of high pressure air impact coal and rock dynamic damage characteristics and the influence law of fracture morphology.(1)A true triaxial test platform integrating functions of high pressure air impact,acoustic emission monitoring and pressure high-speed monitoring was constructed.The stress on the end face of the test block was up to 70 MPa,and the pressure was up to 30 MPa.The movable cavity structure is adopted to realize the movement of the test block with the help of the guide rail,and the limit slot is used to locate the position of the test block,so as to realize the precise three-way stress loading.(2)The impact cracking tests were carried out under six impact pressures(6,8,10,12,14,16MPa)and five drilling uncoupling coefficients(1,1.5,2,2.5,3).The results show that with the increase of impact pressure,the pressure drop rate increases and the crack deflection phenomenon disappears.When the drilling uncoupling coefficient is1.5,the axial crack propagation depth is the largest,while when the drilling uncoupling coefficient is 2,obvious erosion holes are formed at the air flow impact.In conclusion,the optimal impact pressure is 16 MPa,and the optimal drilling uncoupling coefficient ranges from 1.5 to 2.(3)The high pressure air impact cracking test under five impact directions was carried out.According to the pressure curve,the cracking process was divided into five stages,namely,the impact cracking,energy recovery,crack propagation,pressure stabilization and pressure attenuation.Radial and axial cracks will be formed in the hole wall in the impact direction,and the radial cracks will develop to form the impact low-lying area.The axial cracks will expand along the axis and appear deflection phenomenon.The main frequency of AE signal is positively correlated with the impact direction.(4)High pressure air impact cracking tests under five stress difference coefficients are carried out.The results show that with the increase of stress difference coefficient,the peak pressure and pressure attenuation duration increase,while the number of cracks and acoustic emission signal frequency decreases.With the increase of stress difference coefficient,the influence of stress field on the deflection position and Angle of axial fracture is gradually obvious,while the radial fracture surface tends to be flat.(5)The high pressure air impact cracking test of the original rock block was carried out,and the influence law of air pressure on the fracture morphology of the original rock was analyzed.With the increase of air pressure,the test block shows burst failure,and there are impact failure holes in the test block.According to the RA/AF analysis of acoustic emission signal,it is found that tensile failure is the main failure,and shear failure is the second failure.The research results have important guiding value for the application of high-pressure air fracturing technology in the fields of coal seam reflection improvement and rock fracture.Figures 55 Table 13 Parameter 98... |
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